The invention provides a container device for containing and immobilizing articles and workpieces susceptible to mechanical and frictional damage during transport and storage. More particularly, the invention provides a container device with an adjustable volume to hold and immobilize solder spheres used in manufacturing printed circuit boards and other electronic assemblies. The invention also provides a method of immobilizing solder spheres.
Ball grid array (BGA) packages have become the means of choice to house and mount electronic components to larger printed circuit boards and other electronic assemblies. BGA packages typically include a plastic, ceramic or laminated substrate having a specific pattern of pads or recesses on an exposed surface which serve as contact points for mechanically and electrically connecting electronic components, such as integrated circuit chips, to circuit paths of printed circuit boards. BGA packages are mounted to printed circuit boards by surface mount technologies (SMT), including, for example, a process known as reflow soldering. Reflow soldering involves loading solder alloy spheres onto pads or recesses of BGAs and then coupling solder spheres to solder paste applied to specific areas on printed circuit boards which correspond to the array of pads or recesses of BGAs. High temperatures during reflow soldering melt spheres and cause solder to flow around contact points forming mechanical and electrical solder joints between BGAs and printed circuit boards. Accurate placement of solder alloy spheres onto arrays of pads or recesses is, therefore, a critical step in achieving satisfactory mechanical connections and electrical interconnections between integrated circuits housed within BGAs and printed circuit boards.
Automated handling systems, often referred to as pick-and-place systems, are used to load solder alloy spheres to particular soldering sites (pads or recesses) on BGAs. To confirm placement of solder alloy spheres, which can be as small as 0.004 inch in diameter, an automated vision system is often employed to determine if spheres are present at appropriate soldering sites prior to the inception of reflow soldering. An automated vision system rapidly scans an area of loaded solder pads with a high resolution camera. The high resolution camera digitizes the visual data collected to produce a gray level histogram which distinguishes the presence or absence of individual solder spheres by contrasting the bright and shiny surfaces of solder spheres with a darker, more matte background. Such a contrast between the shine of the solder sphere surfaces and the dark background establishes a pass-fail criterion that is used to determine whether a solder sphere is present at a particular site or not. A solder sphere which does not possess sufficient brightness and shine may cause the automated vision system to determine a false fail, indicating that a solder sphere is not present at a particular site, although a solder sphere has, in fact, been accurately placed at the site. The automated vision system may then indicate to an assembling system that a solder sphere is missing, whereby the assembling system may automatically reject the circuit board in question or discontinue production. For such reasons, assemblers and manufacturers of electronic components and systems prefer solder alloy spheres with bright, shiny surfaces in order to avoid erroneous rejection of electronic components and unnecessary production downtime.
It is well known that solder alloy spheres are subject to surface oxidation after manufacture due to an inherent propensity of base metals used in surface alloys, such as tin and lead, to oxidize, causing surfaces of solder spheres to darken. Of particular concern to suppliers of solder spheres is surface darkening produced during transport and storage of solder spheres due to mechanical surface damage, referred to as surface fret corrosion, which exacerbates oxidation. Surface fret corrosion primarily occurs when surfaces of solder spheres rub against each other causing portions of sphere surfaces to be removed or chipped away. Conventional glass or plastic containers are typically filled with solder spheres by mass, rather than volume, which tends to produce an empty or dead space between a bottom surface of a container lid and solder spheres contained therein. The empty space enables solder spheres to tumble about the container and to rub against each other, as well as against the side walls of the container, when the container is agitated during transport and storage. Surface fret corrosion is caused by a phenomenon called xe2x80x9cslip-stickxe2x80x9d, whereby mechanical and frictional energy is produced between rubbing sphere surfaces, causing surfaces to bind up rather than slip past each other. Agitation during transport of the container causes bound surfaces of spheres to suddenly release, chipping away portions of surface alloy. An accumulated loss of surface alloy by surface fret corrosion causes solder spheres to oxidize and darken.
Prior art packaging and containers which include provisions to prevent mechanical damage to articles contained therein, do not include packaging or containers which immobilize solder alloy spheres. Prior art containers are often designed for specific articles and workpieces. U.S. Pat. No. 5,709,301 discloses a storage container having a foam insert disposed in a container lid to hold and store paint brushes and paint rollers. U.S. Pat. No. 4,347,929 discloses a container for storing and transporting explosives having a foam insert attached to a lid with cut-outs to accommodate blasting caps. In more analogous art, U.S. Pat. No. 4,426,675 discloses a carrier box for storing and transporting printed circuit boards which includes strips of electrically-conductive foam material on the bottom and sides of the container to reduce vibration of printed circuit boards during transport.
Therefore, a method of immobilizing solder spheres and a container device for transporting and storing small solder alloy spheres that includes provisions to immobilize solder spheres is desirable to prevent mechanical surface damage and to eliminate surface oxidation of solder spheres.
The invention provides a container device for holding and immobilizing solder spheres used in manufacturing printed circuit boards and other electronic assemblies. The container device includes a hollow containing portion with a first terminal end and a second terminal end with at least one side wall connecting the first terminal end with the second terminal end; a removable sealing mechanism connected to the first terminal end; a piston positioned within the containing portion having a length and a width and of substantially similar shape as the containing portion; and an adjustment mechanism coupled to the piston for adjusting the position of the piston between the first and second terminal ends of the containing portion.
In a first embodiment of the invention, the containing portion and the piston are circular cylinders. The piston is of a sufficient width such that when inserted into the containing portion, the length of the piston presses substantially adjacent to the side wall of the containing portion to rigidly position the piston and to form a seal between the piston and the side wall of the containing portion. In another embodiment, the piston further includes a ridge connected to and surrounding the width of the piston. The ridge is of a sufficient height to exert a pressure of about 1 to about 5 pounds per square inch against the side wall of the containing portion to rigidly position the piston and to form a seal between the piston and the side wall. The ridge is angled at about 45 degrees toward the second terminal end of the containing portion. The angle of the ridge limits movement of the piston toward the first terminal end of the containing portion, thereby preventing movement of the piston toward the second terminal end and preventing removal of the piston from the containing portion once inserted. The piston further includes a centrally-positioned aperture extending the length of the piston to discharge air from the containing portion displaced by the piston when the piston is inserted into and adjusted within the containing portion.
In another embodiment of the invention, the piston further includes a layer of foam connected to a terminal end of the piston in facing relation to the first terminal end of the containing portion. The layer of foam has a sufficient width to exert a linear compression pressure of about 0.5 percent to about 5 percent against the side wall of the containing portion and a mass of solder spheres contained therein. The thickness of the layer of foam is about 0.10 cm to about 1.0 cm. The layer of foam is constructed of an elastomeric closed-cell foam that may be electrostatically dissipative to prevent solder spheres from adhering to the layer of foam. The piston is constructed of a compressible polymeric foam such as, although not limited to, polypropylene, polyethylene, neoprene, polyurethane, polystyrene, polysulfones and polyhalogenated polymers.
The adjustment mechanism includes a rod-shaped mandrel either permanently or removably coupled with the piston that adjusts a position of the piston within the container device upon application of pressure to the mandrel.
The containing portion of the container device is constructed of a suitable material, such as, although not limited to, polypropylene, polyethylene, neoprene, polyurethane, polystyrene, polysulfones and polyhalogenated polymers.
In another embodiment of the invention, a method of immobilizing solder spheres is provided comprising steps of: providing a container device with a hollow containing portion having a first terminal end and a second terminal end with a side wall connecting the first and second terminal ends and a removable sealing mechanism connected to the first terminal end; a piston for insertion into the containing portion having a length and a width and a first and second terminal end, the width of the piston being substantially similar to a width of the containing portion such that the piston conforms to and presses substantially adjacent to the side of the containing portion when inserted into the container device; providing a rod-shaped mandrel coupled to the second terminal end of the piston for insertion of the piston into the containing portion and adjustment of a position of the piston; loading a mass of solder spheres into the container device; inserting the piston into the second terminal end of the containing portion by application of a sufficient pressure to the mandrel; adjusting the position of the piston toward the first terminal end of the containing portion by application of a sufficient pressure such that the piston substantially fills empty volume in the containing portion; and pressing the piston substantially adjacent to or against the mass of solder spheres by application of a sufficient pressure to the mandrel to immobilize solder spheres.
In a version of this embodiment, a further step includes providing a layer of compressible foam connected to the first terminal end of the piston in facing relation to the first terminal end of the containing portion such that the layer of foam exerts a linear compression pressure of about 0.5 percent to about 5 percent against the mass of solder spheres to immobilizing solder spheres.
In another version of this embodiment, a further step includes providing a circumferential ridge connected to and surrounding the piston such that the ridge exerts a pressure of about 1 to about 5 pounds per square inch against the side wall of the containing portion to rigidly position the piston and to form a seal between the side wall and the piston to prevent solder spheres from slipping between the side wall and the piston.